U.S. Pat. No. 1,578,766 discloses a combination vacuum relief valve and pressure relief valve in which both the vacuum relief valve and the pressure relief valve are located below a top cover plate. A bottom cover plate is movable vertically to relieve a predetermined pressure in the tank above atmospheric.
In application Ser. No. 932,796 filed Aug. 11, 1978 a vacuum relief valve is disclosed in which first passageways providing communication between the outside atmosphere and the lower surface of the vacuum relief valve have a cross-sectional area equal to the cross-sectional area of the vacuum relief valve and equal to the cross-sectional area of second passageways providing fluid communication between the top surface of the vacuum relief valve and the inside of the container. In this way the volume of air flow into the container is maximized for a given vacuum relief valve cross-sectional area and pressure drop between the outside atmosphere and the pressure in the tank.
In U.S. Pat. No. 2,169,410 a bottom cover plate is lifted to vent pressure in the tank to atmospheric. A vacuum relief valve is provided in this cover plate which moves downwardly to allow air to enter the tank, and which is weighted to control the vacuum relief pressure. However, the volume of air flow is not maximized in this construction.